Physiological studies on cells of Klebsiella aerogenes suggest that an enzyme, glutamine synthetase, also functions to regulate transcription of genes coding for enzymes involved in nitrogen metabolism. In the case of the hut genes, which are necessary for histidine degradation, it has been demonstrated directly, in a purified transcription system, that highly purified glutamine synthetase is a positive control element which activates transcription of the hut genes. We believe that glutamine synthetase also acts to repress transcription, that the non-adenylylated form of the enzyme represses transcription of the gene for glutamate dehydrogenase and the adenylylated form of the protein represses transcription of the structural gene (glnA) for glutamine synthetase. These ideas will be tested directly by assaying by RNA-DNA hybridization the level of glnA-specific mRNA in wild-type and mutant cells of K. aerogenes grown under various conditions, by examining the ability of glutamine synthetase protein to repress transcription of the glnA DNA in a purified transcription system, and by examining in whole cells the regulatory properties of glutamine synthetase from various bacteria. These studies should augment the present knowledge of regulatory mechanisms of transcription and lead to a valuable probe for future studies on protein nucleic acid interactions.